The present inventions relate generally to spread spectrum communications, and more particularly to extracting time from spread spectrum signals, for example from Global Positioning System (GPS) signals, with mobile wireless communication devices, and methods therefor.
GPS enabled cellular handsets will likely provide near term solutions for complying with the E-911 location determination mandate of the Federal Communications Commission.
The existing GPS satellites transmit a C/A code (having a length of 1023 bits) and a 50 bit per second (BPS) navigation data message from which time can be determined on an L1 channel signal at 1575.42 MHz. The L1 channel also includes a P/Y military signal. The existing GPS satellites also transmit the P/Y military signal on an L2 channel signal at 1227.6 MHz.
Military receivers are capable of demodulating the P/Y signal and measuring the delay between the L1 and L2 channel signals, which permits removal of ionospheric delay error, thus providing measurement accuracies under 5 meters. In contrast, measurement accuracies based only on the L1 channel signal are limited to approximately 20-25 meters, due largely to ionospheric delay.
Demodulation of the 50 BPS navigation data message is possible down to a signal detection level of approximately 30 dB-Hz, but this is generally too high for most GPS enabled cellular handsets, which require a lower signal detection level of about 20 dB-Hz or less. In GPS enabled cellular handsets, it is desirable to extract precise time directly from GPS spread spectrum signals without having to demodulate the navigation data message.
In the past, various proposals have been made to modify GPS transmission signals and in particular the L2 channel signal to better accommodate civil/commercial applications when the GPS satellites are upgraded.
A current GPS signal modification proposal described in the recently published xe2x80x9cL2 Civil Signal (L2CS) Design Plansxe2x80x9d, for example, includes a new C/A code on the L2 channel. The generation of the proposed new L2CS signal is based on a bit-by-bit multiplexing of long and short chip codes, one of which is an integer multiple of the other. The long and short chip codes of the L2CS proposal are both longer than the existing 1023 bit C/A Code and provide up to 45 dB of cross correlation protection, the dynamic range within which most cellular communication devices operate.
The proposed new L2CS signals also extend the time ambiguity, or window of certainty, to 1.5 seconds. The extended time ambiguity is a substantial improvement over that of the existing GPS C/A code, which has a time ambiguity of 1 millisecond, or 20 milliseconds if one considers the edges of the 50 BPS navigation data message.
Prior Art FIG. 1 illustrates a schematic circuit block diagram for implementing the new L2CS GPS signal. The circuit includes a 767,250 Chip Code Generator for generating a 767,250 long chip code and a 10,230 Chip Code Generator for generating a 10,230 short chip code. Prior art FIG. 2 illustrates the long chip code as an integer multiple of the short chip code, which repeats exactly 75 times for each 767,250 long chip code. Although time may be known precisely during any interval T, corresponding to the 1.5 second period of the long chip code, there is no way to determine readily in which repeating interval of the long chip code time has been measured, i.e., at interval 1, 2, 3 . . . ?
The bit-by-bit multiplexing operation of the proposed L2 Civil Signal allows rapid handover from the 10,230 chip short code to the 767,250 chip long code by one of two alternative methods. In one handover scheme, in Prior Art FIG. 1, the 10,230 chip short code is mixed with an L-5 like data message including precise time, similar to the L1 channel GPS signal. Cellular handsets, however, may be unable to demodulate this signal. Also, the navigation data message with precise xe2x80x9cTime of Weekxe2x80x9d bits and other information, including for example, satellite orbit ephemeris, is transmitted only every 6 seconds, which extends the time and battery power required by cellular handsets to obtain time.
In another handover scheme, the receiver directly acquires the 10,230 short chip code and then acquires the 767,250 long chip code by trial and error correlation, in which all 75 possible alignments of the long chip code relative to the short chip code are tried. In this latter scheme, when the receiver locks onto the 767,250 long chip code, time is known precisely to a resolution of 1 short or long code chip time (about 1 microsecond), but absolute time remains unknown.
In some applications, including cellular handsets where it is desired to obtain time without having to demodulate a periodically transmitted navigation data message, 1.5 seconds may be insufficient to extract time from the GPS signals. What is desired in these and other applications is a method of extracting precise time directly from spread spectrum signals without having to demodulate the navigation data message.
The various aspects, features and advantages of the present invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description of the Invention with the accompanying drawings described below.